Core Body Temperature

The Circadian Temperature Cycle

Core body temperature follows a precise 24-hour circadian cycle that is tightly linked to the sleep-wake cycle:

Morning (6am–10am): Core temperature rises from its nadir, helping you wake up and become alert. The rising temperature suppresses melatonin and promotes cortisol release.

Daytime (10am–6pm): Core temperature reaches its peak (~37.4°C) in the late afternoon. This is when physical and cognitive performance is highest.

Evening (6pm–10pm): Core temperature begins declining, triggered by melatonin release and peripheral vasodilation (blood flow to hands and feet to radiate heat). This decline signals the brain that sleep onset should begin.

Night (10pm–4am): Core temperature continues dropping, reaching its nadir (~36.4°C) at approximately 4am. This is the period of deepest sleep; the body's most intensive physical recovery.

Pre-dawn (4am–6am): Core temperature begins rising again, preparing the body for waking.

This temperature rhythm is not a side effect of sleep; it is the control signal. The temperature drop causes sleep; sleep does not cause the temperature drop.

Czeisler, C.A. and Gooley, J.J. (2007). Cold Spring Harbor Symposia. | Walker, M. (2017). Why We Sleep.

The 1–2°C Drop: The Key to Deep Sleep

The core temperature drop of 1–2°C (from ~37.4°C to ~36.4°C) is the physiological trigger for deep sleep. Here is how it works:

1. As ambient light decreases in the evening, the suprachiasmatic nucleus (SCN) signals the pineal gland to release melatonin. 2. Melatonin triggers peripheral vasodilation; blood vessels in the hands and feet widen, directing warm blood to the skin surface to radiate heat away from the core. 3. This peripheral heat loss causes the core temperature to drop by 1–2°C. 4. The preoptic area of the hypothalamus detects this temperature drop and triggers the cascade of events that initiate deep sleep.

If this temperature drop is prevented; by heat-trapping bedding, a warm environment, or hormonal disruption; deep sleep is delayed or incomplete. Research shows: • A 1°C increase in sleep surface temperature reduces deep sleep by 25%. • Sleep surface temperatures of 16–19°C increase slow-wave sleep by up to 32%. • The body generates approximately 80 watts of heat during sleep; all of which must dissipate through the skin surface.

This is why the sleep surface (bedding) is the critical variable for core temperature regulation during sleep.

Harding, E.C. et al. (2019). Frontiers in Neuroscience. | Sleep Medicine Reviews (2019).

How to Support Healthy Core Temperature During Sleep

To support the natural core temperature drop needed for deep sleep:

1. Use cooling bedding: CoolTouch® Performance Comforter (Q-Max 0.38) actively absorbs body heat at the skin surface, facilitating the core temperature drop. Clinical data: 32% more deep sleep.

2. Use a cooling pillow: The CoolCloud® Pillow manages head/neck temperature. The brain's thermoregulatory centre is in the hypothalamus (in the head), and the head dissipates 25–30% of body heat.

3. Use TENCEL™ Lyocell sheets: 50% more breathable than cotton, facilitating heat transfer from skin to environment.

4. Set AC to 19–21°C: Provides the ambient temperature that supports the body's natural cooling process.

5. Maintain consistent sleep timing: The circadian temperature rhythm depends on consistency. Irregular sleep schedules disrupt the temperature cycle.

6. Get morning light exposure: Morning sunlight helps set the circadian rhythm, which controls the temperature cycle.

7. Avoid alcohol before bed: Alcohol disrupts the temperature rhythm and suppresses deep sleep.

By supporting the body's natural thermoregulatory process, you enable the core temperature drop that is the physiological trigger for deep, restorative sleep.

Sleep Lab™ Clinical Study, 2023. | Harding, E.C. et al. (2019).